This invention relates to a system for transmitting a signal in packet form, in which a signal is required to be transmitted while maintaining real-time correspondence between transmission and reception.
With the spread of computers and the development of processing techniques for digital signals, much attention has been directed to data communications for processing information by on-line operation, in the combination of communication systems and data processing systems. Specifically, in small scale communication systems such as for private communication within a premises such as government or municipal offices, companies, etc. a communication system in packet form utilizing communication cables such as coaxial cable has attracted public attention because of its high economic efficiency, reliability and transmission efficiency.
According to such a communication system, communication cables for bi-directional transmission are installed in the building or the like and a number of stations (personal stations) are connected with the cables. A message is divided into data blocks, each including 1000.about.2000 bits at the maximum, and is transmitted from each station. A header such as an address and serial number is added to the message. In this communication system, the network constitutes a passive transmission medium with no control function therein, as all of the required control functions are completely distributed to the respective personal stations. Therefore, each station begins to transmit a message after confirming a vacancy in the transmission path, and if the packet sent from a certain station should collide with another packet from other station, transmission of the packets are stopped at both stations. Those stations having transmission of the packets therefrom stopped try again to transmit the messages after the lapse of a random waiting period.
In the communication system as mentioned above, it is possible for users at the respective locations not only to gain access to the same computer, but also to commonly utilize hardware such as the memory and software (programs), which are distributed to the respective locations. In other words, devices including high speed or high accuracy printers and files with large capacity that were centralized on the central large scale computer and operated with the aid of a TSS (time sharing system) can be distributed to the respective locations and hence utilized in a distributed manner. Accordingly, such a communication system leads to a saving of resources and an improvement in working efficiency, as well as permits the development of large scale software systems due to the versatility of the programs and data. Furthermore, there is no use priority of transmission paths among the respective users (personal stations), such that they stand on an equal basis in the communication system. As a result, no hierarchy with main-slave relations among the stations is produced as often experienced in other communication systems, so that it becomes possible to communicate between any two stations which have been connected to each other. In addition, since the transmission paths are comprised of coaxial cables or similar completely passive circuits, a high reliability system can be made with ease.
As stated above, the foregoing communication system has various advantageous features, but, at the same time, is accompanied by a problem in that it is not suitable for the transmission of a signal whose transmission quality is greatly influenced unless the maintenance of real-time correspondence between transmission and reception is achieved. More specifically, the above-described communication system is disadvantageous in that a delay time is necessarily caused in the transmission process because data transmission in packet form is carried out utilizing a vacant time in the transmission path, and further because the delay time may vary greatly due to a collision between packets. This disadvantageous tendency naturally becomes more significant with increasing working efficiency or usage of the transmission paths of the system.
Therefore, voice communications have not generally been carried out by use of packet-form transmission systems up to now. If used, the application of such systems has been limited to uni-directional communications such as an answerphone or a dictation system in which only the content of a memory is transmitted. In other words, for the purpose of performing conversational voice communications, it has been required to abandon data transmission in packet form and the various advantages thereof, or provide a separate transmission path exclusively for voice communications.